Camera turns on and works but has constant error cf and when you try to format the card reads error 99

check for bent CF pins

error 99 can most times be remedied by taking a standard pencil eraser and carefully cleaning the gold contacts at the back of the lens and the gold maching contacts in the lens opening of the camera... BE CAREFUL not to let eraser crumbs into your camera... keep it clean... reset the battery and give it a whirl.... did it solve the 99 issue?
for the CF it refers to the cf card... here's what I've found on this:
Canon reports that this error code may appear on your digital camera’s LCD (liquid-crystal display) when there is an error reading from or writing to the inserted CF (CompactFlash) card. This error code occurs most often after a user has tried to eject a CF card before the camera has finished writing data to the card.Solution:
Canon suggests that you first try to use a different CF card. If you do not see the error message when you use the new CF card, the first card is probably damaged.

Before you take the next step, drag the files from the CF card to a location on your hard drive to back up the files. Next, you can use Windows’ ScanDisk utility to check the CF card for errors. If you use a Mac, you can use Disk Utility (in Applications\Utilities\) in OS X.

If ScanDisk or Disk Utility fails to resolve the CF card problem, insert the CF card in your camera again. Then format the CF card. On most Canon cameras, you can do this by pressing the Menu button, selecting the Format tab, highlighting Format, selecting Format, and choosing Yes. Make sure you have backed up the data on the card before you do this. Formatting the card will delete the images from the card. After you format the card in your camera, use ScanDisk or Disk Utility to check the CF card again. If you still encounter errors while using ScanDisk or Disk Utility, you should discard the CF card because it is damaged.

If the error still occurs after following the steps above, your camera may need repair. Contact Canon’s technical support for further assistance.

hope this solves your issues....
let me know
K

I Googled the error code you're getting, and found a list of things to try and steps to attempt directly from Canon. Try out some of the stuff below, it should help.

To isolate the cause of the issue, we suggest that you do the following:
1. Turn off the camera.
2. Remove the lens, battery, and CF card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert a fully charged battery, and turn on the camera.
5. Depress the shutter button as you would to take a picture.
Does the “ERR 99′ message appear? If it does, then the camera should be serviced. If it does not, then please proceed to the next set of steps:
1. Turn off the camera.
2. Insert the CF card.
3. Turn on the camera.
4. Format the CF card in the camera.
5. Depress the shutter button as you would to take a picture.
Does the “ERR 99′ message appear? If so, then the CF card is the most
likely source of the issue. Try using a different card.
If the message does not appear, please complete the following steps:
1. Turn off the camera.
2. Clean the lens contacts by gently rubbing them with a pencil eraser
or soft cloth. Be careful that you do not let any debris fall into the
camera body.
3. Reattach your lens.
4. Turn on the camera.
5. Depress the shutter button as you would to take a picture.
If the “ERR 99′ message only appears when one particular lens is attached, then that lens should be examined by a service technician.
If you see the “ERR 99′ with a different Canon lens attached, then the camera should be serviced.

Nope, this is not card anymore.

As you know CF is card failure, sometimes this cause to start error 99 , that is lens error , even if there is no problem with the lens.

This happens because the built in software in the camera can get messed up after an error CF.

You fix the problem calling Canon, they will guide through upgrading the firmware in your camera.

First ensure that you are using the camera menu and command to format the new card, and that new cards are properly inserted into the machine.

Call Canon support to get a firmware upgrade.

Not all CF cards are created equally, so yes, it could be your batch of cards. To determine if the error is CF related, and not anything else, try the following steps which are used to diagnose err 99's

To isolate the cause of the issue, do the following: -

1. Turn off the camera.
2. Remove the lens, battery, and CF card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert only a fully charged battery, and turn on the camera.
5. Depress the shutter button as you would to take a picture.


Does the “ERR 99′ message appear? If so, the camera may be faulty and requires a service. If it does not, then: -

1. Turn off the camera.
2. Insert the CF card.
3. Turn on the camera.
4. Format the CF card in the camera.
5. Depress the shutter button as you would to take a picture.


Does the “ERR 99′ message appear? If so, then the CF card is faulty. Try a different card. If the message does not appear: -

1. Turn off the camera.
2. Clean the gold contacts on the lens by GENTLY rubbing them with a pencil eraser or soft cloth. Keeping the camera face pointing down, also clean the gold contacts on the camera, being careful that you do not let any debris fall into the camera body.
3. Reattach your lens.
4. Turn on the camera.
5. Depress the shutter button as you would to take a picture.
If the “ERR 99′ message only appears when one particular lens is attached, then that lens should be examined by a service technician.
If you see the “ERR 99′ with a different Canon lens attached, then the camera is faulty and should be serviced.

Not sure about the pattern of errors, but the advice for error 99 diagnosis is to try the following diagnosis steps to isolate the cause of the issue:-
1. Turn off the camera.
2. Remove the lens, battery, and CF card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert only a fully charged battery, and turn on the camera.
5. Depress the shutter button as you would to take a picture.

Does the “ERR 99" message appear? If so, the camera may be faulty and requires a service. If it does not, then: -
1. Turn off the camera.
2. Insert the CF card.
3. Turn on the camera.
4. Format the CF card in the camera.
5. Depress the shutter button as you would to take a picture.

Does the “ERR 99" message appear? If so, then the CF card is faulty. Try a different card. If the message does not appear: -
1. Turn off the camera.
2. Clean the gold contacts on the lens by GENTLY rubbing them with a pencil eraser or soft cloth. Keeping the camera face pointing down, also clean the gold contacts on the camera, being careful that you do not let any debris fall into the camera body.
3. Reattach your lens.
4. Turn on the camera.
5. Depress the shutter button as you would to take a picture.

If the “ERR 99" message only appears when one particular lens is attached, then that lens should be examined by a service technician.
If you see the “ERR 99" with a different Canon lens attached, then the camera is faulty and should be serviced.

Error 99 can mean many things. Have you installed the latest firmware for the camera (version 1.1.1)? If you don't have, download it here (for equipment purchased in USA or Puerto Rico):

http://web.canon.jp/imaging/eosd/eos40d/eos40d-firmware-e.html

If you have the latest firmware, try this:

1. Switch off the camera and remove the CF card and battery
2. Keep the camera idle for 30 min
3. Clean all the contacts of the camera (battery contacts, lens contacts etc) with an eraser. Do this keeping the camera facing down so that no debris fall into the camera.
4. Attach the lens and insert the battery.
5. Switch on the camera
6. Insert the CF card
7. If the error persists, format the card again
8. If this does not work, try using another CF card.
   

Does the camera do this with any CF card? If you've tried another CF card with the same problem, check in the slot that the CF card fits into with a strong light to make sure no pins are bent and touching each other.

Also, was the card formatted with a Windows 2000 or Windows XP system? If you format with Windows, make sure you specify FAT32 as the file system (rather than NTFS).

Err 99 is a 'catch-all' error message that is non-specific. It indicates that the camera detects an error within its operating system, but cannot determine the specific reason.

In summary, try another card. If you get the same error, and no pins are bent, you probably need to have the camera serviced.

here is a link for further steps to take on Error 99: Canon info

The article refers to the 30D, but the same troubleshooting will work for your camera too

My canon xti had the same reading cf card error 99.
Since my camera is out of warranty. I had to find a way to fix it. After reading all possible solutions in this site, I looked at the pins (that used to connect to the CF card) and found out that one pin in the middle bent. That caused the problem. I used the tiny screw driver to bend that pin back to the original position. Inserted the card back and it worked. I felt so happy!!!! (Please make sure to have the battery removed while fixing...to be saved.)